Taxonomic Revision of Ningshan Odd-Scaled Snake, Achalinus ningshanensis (Serpentes, Xenodermidae), with Description of a New Subspecies from Western China

<i>Achalinu ningshanensis</i> (Yang, Huang, Jiang, Burbrink, and Huang, 2022) was first described in Ningshan County, Shaanxi Province, China in 2022, based on seven female specimens. In this study, based on phylogenetic analyses using mitochondrial 12S ribosomal RNA (<i>12S</i>), 16S ribosomal RNA (<i>16S</i>), cytochrome c oxidase subunit 1 (<i>CO1</i>), cytochrome b (cyt <i>b</i>) gene fragments, and morphological examinations of specimens, we revise the taxonomic status of <i>A. ningshanensis</i>, and provide additional data on this species. The molecular phylogeny indicated that <i>A. ningshanensis</i> is nested in a highly supported monophyletic group, forming a sister taxon to <i>A. spinalis</i>, and is divided into two well-supported lineages, A and B, with an uncorrected <i>p</i>-distance between lineages from 3.6 to 4.3% for <i>CO1</i>. Therefore, we proposed that Lineage B from western Sichuan and southwestern Shaanxi is a new subspecies, <i>Achalinus ningshanensis occidentalis</i> ssp. nov., and Lineage A from southern Shaanxi and northeastern Sichuan is allocated as <i>Achalinus ningshanensis ningshanensis</i>. Morphologically, the new subspecies can be distinguished from its congeners, especially from <i>Achalinus ningshanensis ningshanensis</i>, by the following characteristics: (1) the tail is relatively short, with a TAL/TL ratio of 0.202-0.226 in males, and 0.155-0.178 in females; (2) there are two pairs of chin-shields; (3) there are 21-22 maxillary teeth; (4) the length of the suture between internasals is significantly shorter than that between prefrontals, with an LSBI/LSBP ratio of 0.502-0.773; (5) there are six supralabials, with the fourth and fifth in contact with the eye; (6) there are five to six infralabials, and the first to third or fourth touches the first pair of chin-shields; (7) there is one hexagonal loreal, with an LorH/LorL ratio of 0.612-1.040; (8) the two anterior temporals are in contact with the eye; (9) there are 155-160 ventrals in males, and 165-174 in females; (10) there are 60-65 subcaudals in males, and 49-53 in females, which are not paired; and (11) the dorsum is iridescent and uniformly charcoal black, lacks a longitudinal vertebral line, and has a dark brown or dark gray ventral area.